The use of individualised volume holograms as overlays for security documents has been demonstrated by groups such as Bundesdrückerei. The current German passport and ID card comprise volume hologram overlays. Ver-tec Security Systems Ltd described the “Biometrigram system”—see WO2005/034019 which provided a holographic film recording of biometric features suitable as a verification device for high-security documents. Volume reflection holograms can be recorded in a colourless thin layer, perhaps only a few microns thick and supported by a thin carrier film, and as such are highly suitable for application as an overlay to a security document. Further background prior art can be found in WO2010/046687.
Human portraits have regularly been used as the subject for holographic security labels but embossed holograms are by nature, each an identical facsimile of the master image. One of the earliest human portraits used in embossed holography was the well known image representing William Shakespeare produced by Applied Holographics for the protection of an APACS bank card (the “Bard card”.). This was an embossed holographic stereogram in full colour, and its image included animation as the viewer moved left to right or tilted the hologram correspondingly; the subject smiling and appearing to speak as the viewing angle changed. Embossed holograms typically comprise an aluminised film, and as such are not suitable for use as an overlay. Therefore, security applications such as the British passport data page have been developed where a thin coating of high reflective index material, such as zinc sulphide, has been used in place of the aluminium layer to enable a relatively transparent layer so as to permit a view of the printed data on the paper directly behind the hologram; but this “h.r.i.” layer still retains mirror like qualities which do not permit all of the incident light to be transmitted through the layer without reflection.
Using modern materials, reflection volume holograms have the capability to provide a full colour image comprising, for example, a facial portrait in three dimensions; the whole device contained in a thin, transparent film layer. Animation is possible in the same way as has been achieved in embossed holography as described above.
One of the problems of the rainbow hologram method for embossed holography is that the tilting of the hologram results in a change of colour in the perceived image as each of the colour components cycles through a full range of rainbow colours. This effect is not conducive with the viewer's perception of a recognisable facsimile of a real person. However, the rainbow effect is not the case with reflection holography, as the volume grating effectively acts as a reflective wavelength filter, which is not very susceptible to angular change in the viewing condition.
Another alternative technique for display of three-dimensional images is the use of lenticular displays. An example of a mass produced lenticular portrait is the (limited edition) cover of the CD album “Hours.” by David Bowie, in this case a purely decorative device. However, in the British driving licence there is a lenticular image used as a security device which displays driver detail/date information when the plastic licence is tilted.
The thin layer of the holographic film assembly in photopolymer or silver halide is far more convenient to apply to a document or product than a lenticular three-dimensional device whose impressed relief plastic lens device is thicker than a holographic recording film layer. The image quality and resolution of the holographic image is far higher than the maximum resolution of a lenticular image. Typically a holographic stereogram may comprise for example 60 to 100 channels of stereographic or animation information.
The German identity cards made by Bundesdrückerei, include a personalised holographic portrait. But this is two-dimensional and the whole overlay is predominantly in a single colour, except for a small area of the surface which has been chemically treated so as to show a second reflected colour. There is a separate printed ID portrait on the card, which can be compared with the hologram. But monochromatic reflection holograms have a limited security value, because for many years films such as Agfa Holotest recording materials have been available and it is only since the availability of modern high resolution ultra-fine grain silver halide and panchromatic photopolymer recording materials that more complex full colour reflection volume holograms have become a realistic possibility for widespread use.
One of the inherent problems of reflection holography is that, despite the fact that individual monochromatic reflection volume gratings are capable of diffraction efficiency approaching 100%, the application to paper or plastic card documents, which are in many cases not specifically designed in such a way as to present the holographic image in the best circumstances, is often prone to dilute the visual effectiveness of full colour (tri-stimulus) holograms.
For example, a reflection hologram which is highly efficient may be laminated to a document printed upon white paper with a limited quantity of pigment overprint. Thus, despite the high diffraction efficiency of the overlaid reflection hologram, the highly reflective white paper might well cause a dilution of the effectiveness of the hologram by effectively reducing the image contrast therein, since the bright paper is simultaneously visible to the viewer as he or she views the transparent hologram film layer. For example, a red highlight in the holographic image could reflect a high proportion of the red component of the incident light, but the remaining components of the white light will then be transmitted through the hologram bearing film and will be reflected diffusely from the paper below.
Whereas the hologram itself has a narrow reflectivity spectrum, for example reflecting strongly a bandwidth of only say 10 nm, the paper will tend to reflect at a slightly lower of efficiency in that particular wavelength; depending upon the paper quality and its corresponding content of highly reflective white material (such as Titanium Dioxide, Baryta [gelatin-barium sulphate], and the like) but will reflect a high proportion of adjacent wavelengths, which will have the effect of reducing the contrast experienced by the viewer of the reflection hologram.
We are familiar with the experience of viewing scenes recorded photographically or in conventional printing format where the principle of subtractive colour is utilised; but in the case of a hologram, we find a new phenomenon wherein the image contains the usual highlights, but the recording medium, an assembly of a colourless photosensitive layer typically coated upon a transparent film substrate, such as PET, does not contain the absorbent pigments which usually provide stark contrast to the highlights and thus increase the effective “Gamma” of the recorded image.
As an example of this problem, the pupil of the subject's eye in a high quality photographic portrait may importantly be of a very high density black, typically with a strong pinpoint highlight. Details such as this feature, or the shadow details which provide exceptionally important ‘depth cues’ in photography or printed may easily be disguised or lost by the existence of a pale coloured substrate upon which the translucent holographic film may be overlaid.
This phenomenon of contrast reduction has often also been expressed with regard to embossed holograms, whose silver (aluminised) backing layer is so shiny as to be regarded as having mirror-like qualities. But these specular mirror-like qualities are active only at a precise viewing angle, where the equal angles of incident and reflected light from the silver layer fortunately do not coincide with the angle of diffracted light from the hologram when illuminated at its defined reference angle. Noise within the holographic recorded may also tend to manifest itself as haze which will again deteriorate the effective gamma of the image. Ironically, as a result of the problems described where reduced gamma causes difficulty in the perception of the image detail there is often a requirement expressed to increase the “brightness” of the holographic itself, which frequently introduces further noise into the image and thus actually reduces the colour saturation which is a requirement of the desire for realism.
One solution to this problem of lack of dynamic range in a reflection volume holographic image, which has often been used, is to laminate the hologram layer onto a black under-layer. The result is that the apparent contrast in the holographic image is improved; the absorbent backing has a very high black density and the reflectivity of the hologram grating, which can be of the order of 90% of the incident light at a particular narrow band of wavelengths, is enhanced when viewed against a background which is highly absorbent of zero order light transmitted though the diffracted layer, and effective in absorbing all incident light which does not correspond to the reflective spectrum of the diffraction grating.
However this black under layer has the unsatisfactory effect of making the hologram appear as an undesirable black patch within the document. The present technique avoids this disadvantage in a particularly effective way.